Presently, contact lenses are constructed of certain types of plastic materials. For instance, hard or essentially semirigid contact lenses are predominantly made from polymethylmethacrylate. However, this material is not completely satisfactory because of its poor gas permeation characteristics. This, in turn, induces edema on the surface of the cornea in the region covered by the contact lens and thereby resulting in complications and poor vision.
In order to overcome this problem of insufficient gas permeability, it has been suggested to employ cellulose acetate butyrate as a contact lens. Although contact lenses prepared from cellulose acetate butyrate have improved gas permeability characteristics, satisfactory optical and transparency properties, are nontoxic and are physiologically compatible with the corneal tissue which they contact, such suffer from warping. This warping may be due to instability caused by internal stress as discussed by Pearson, Journal American Opthamology Association, 49(8), pages 972-929 (1978). An attempt to ameliorate this warping problem by exposing the lens blanks to an elevated temperature for a predetermined length of time prior to machining and grinding of the lens has been suggested. This heat treatment does eliminate warping to some extent. Post curing or annealing molded plastics in this manner is a commonly used method for relieving molded in stress.
In addition, it is desirable to include a plasticizer in cellulose acetate butyrate when employing such as a contact lens, in order to facilitate processing of the material and to increase the flexibility and comfort of the lens. However, the presence of plasticizers can also lead to instability of the lens such as by leaching out into aqueous media or by absorbing water. Moreover, the presence of the plasticizer itself, due to softening, results in some degree of warpage and long term creep.